This invention relates to optical devices, especially devices comprising particles.
Nanoparticles are particles of very small size, typically less than 100 nm across. The preparation of well-defined nanoparticles via colloid chemistry was demonstrated at least as early as the 1980s. A review of the current technology in this field is given in M P Pileni, Langmuir, 13, 1997, 3266-3279. There are three principal established routes for the formation of nanoparticles: a microemulsion route, a sol-gel route and a high temperature process used principally for semiconducting nanoparticles such as CdSe.
The synthetic principles of microemulsions have been widely described in the literature. For recent work see for example: F. J. Arriagada and K. Osseo-Asare, xe2x80x9cSynthesis of Nanosized Silica in Aerosol CT Reverse Microemulsions,xe2x80x9d Journal of Colloid and Interface Science 170 (1995) pp. p17; V. Chhabra, V. Pillai, B. K. Mishra, A. Morrone and D. O. Shah, xe2x80x9cSynthesis, Characterization, and Properties of Microemulsion-Mediated Nanophase TiO2 Particles,xe2x80x9d Langmuir 11 (1995) pp. 3307-3311; H. Sakai, H. Kawahara, M. Shimazaki and M. Abe, xe2x80x9cPreparation of Ultrafine Titanium Dioxide Particles Using Hydrolysis and Condensation Reactions in the Inner Aqueous Phase of Reversed Micelles: Effect of Alcohol Addition,xe2x80x9d Langmuir 14 (1998) pp. 2208-2212; J. Tanori and M. P. Pileni, xe2x80x9cControl of the Shape of Copper Metallic Particles by Using a Colloidal System as Template,xe2x80x9d Langmuir 13 (1997) pp. 639-646. A microemulsion is a sufficiently thermodynamically stable solution of two normally immiscible liquids (for example, oil and water) consisting of nanosized droplets (or cores) of one phase in another xe2x80x9ccontinuousxe2x80x9d phase, stabilised by an interfacial film of a surfactant with or without a co-surfactant. Examples of surfactants include ionic ones such as Aerosol OT and cetyldimethylethylammonium bromide, and non-ionic ones such as the polyoxyethylene ether and ester surfactants. Examples of co-surfactants include medium to long alkyl-chain alcohols such as 1-hexanol. Examples of oils include hydrocarbons such as cyclohexane and isooctane. The surfactant and co-surfactant molecules reduce the interfacial tension so that stable dispersions can be formed.
Forming nanoparticles by the microemulsion route typically involves preparing a reaction mixture as a water-in-oil reverse micellar system using a ternary phase mixture containing high oil and surfactant contents, but low water content. This allows discrete but thermodynamically-stable nanometer-sized xe2x80x9cwater poolsxe2x80x9d or xe2x80x9cwater coresxe2x80x9d to develop in the reaction mixture. In a typical water-in-oil microemulsion, the water cores are around 1 to 10 nm in diameter. One reactant for the nanoparticle formation can be initially housed in these water cores. The second reactant can subsequently diffuse into and react inside these xe2x80x9cnano-reactorsxe2x80x9d in the normal course of microemulsion dynamics. In this way, microemulsions provide a versatile route to the controlled synthesis of a wide array of oxide and non-oxide types of nanoparticle. In the water pools a metal salt can be reduced to the free metal, or metathesis reactions can be included, to obtain a controlled nucleation and growth of the desired nanoparticle material. The surfactant also acts as a coating to prevent unwanted flocculation (agglomeration) of the growing particles. Many of the fundamental principles governing such micellar chemistry, such as reaction rate and final growth size, are still largely unknown. Most experiments are done by trial-and-error and the data interpreted empirically.
Much of the work on nanoparticles has concentrated either: (i) on demonstrating that nanometer-sized particles have indeed been created (for instance by using transmission electron microscopy (TEM) or ultraviolet-visible (UV-Vis) spectroscopy); or (ii) on subsequently sintering the nanoparticles to prepare a sintered body. This work has involved relatively crude techniques for handling the nano-sized material. For aspect (i) there has generally been no need to isolate or further manipulate the nano-size material. For aspect (ii) the formed material has typically been recovered from the emulsion by bulk precipitation upon addition of a destabilising solvent, or by vacuum removal of the reaction solvent. The material is then sintered at high temperatures to obtain the desired nano-grained article after xe2x80x9cburning offxe2x80x9d of the surfactant coating. Since the particles are to be sintered into a solid mass there is no need to counteract their tendency to aggregate.
Some work has been done on other uses for nanoparticles. S Carter, J C Scott and P J Brock, Appl. Phys. Lett, 71, 1997, 1145-1147 describe the use of polydispersed TiO2, SiO2 and Al2O3 nanoparticles in the form of a blend in polymer LED devices with the aim of enhancing the forward emission of light generated in the LED and/or improving carrier injection and recombination. The route by which the nanoparticles are obtained is not described, but the particles are described as having relatively large sizes: 30 to 80 nm, especially in comparison to the device thickness of 110 nm. It appears from the presence of light scattering that the nanoparticle material suffers from agglomeration. Thus the nanoparticular nature of the material cannot be fully exploited.
In some other works, for instance V. L. Colvin, M. C. Schlamp and A. P. Alivisatos Nature 370, 6488 (1994) xe2x80x9cLight-emittng-diodes made from cadmium diselenide nanocrystals and a semiconducting polymerxe2x80x9d, the use of CdSe nanocrystals as a form of transport layer (deposited neat either by spin-coating or electrostatic self-assembly) in a multilayered device with organic light-emitting polymers has also been described. Another reference relating to nanoparticle polymer composites is J. Schmitt, G. Decher, W. J. Dressick, S. L. Brandow, R. E. Geer, R. Shashidhar, and J. M. Calvert, xe2x80x9cMetal nanoparticle/polymer superlattice films: fabrication and control of layer structure,xe2x80x9d Adv. Mater., vol. 9, pp. 61, 1997.
Organic materials are used for a wide range of applications, including the formation of light emissive devices (see PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference). There is often a need to tune the properties of such an organic material. For example, in the manufacture of optoelectronic devices there is a need for control over various properties of the materials to be used, including conductivity (and/or mobility), refractive index, bandgap and morphology. Some examples of known techniques for tuning various properties are as follows:
1. Conductivity. This has been tuned by adding a chemical compound that acts as a donor or acceptor (namely an electronic dopant), see C. K. Chiang, C. R. Fincher, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, and A. G. MacDiarmid, xe2x80x9cElectrical Conductivity in Doped Polyacetylene,xe2x80x9d Phys. Rev. Lett., vol. 39, pp. 1098-1101, 1977.
2. Charge generation and photo-voltaic response. This has been tuned by blending two materials with appropriate electronic levels so that electrons prefer to reside on one and holes on the other. The blends have been either of two organic materials such as polymers (J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, and A. B. Holmes, Nature 376, 498 (1995), xe2x80x9cEfficient photodiodes from interpenetrating polymer networksxe2x80x9d) or of organic material with a nano-particle (N. C. Greenham, X. G. Peng, and A. P. Alivisatos, xe2x80x9cCharge separation and transport in conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity ,xe2x80x9d Phys. Rev. Bxe2x80x94Cond. Matt., vol. 54, pp. 17628-17637, 1996) to achieve exciton dissociation at the interface.
3. Band-gap and emission colour. This has been tuned by mixing organic compounds in the form of blends or co-polymers (see co-pending UK patent application number 9805476.0).
4. Scattering. Highly aggregated or very large size particles have been blended into polymers to take advantage of multiple internal light-scattering so as to increase the effective length the light travels within the polymer and hence, enhance amplified stimulated emission processes (F. Hide, B. J. Schwartz, M. A. Diazgarcia, and A. J. Heeger, xe2x80x9cLaser-emission from solutions and films containing semiconducting polymer and titanium-dioxide nanocrystals,xe2x80x9d Chem. Phys. Lett., vol. 256, pp. 424-430, 1996). Scattering might have also been used to enhance external efficiency of LEDs (S Carter, J C Scott and P J Brock, Appl. Phys. Lett, 71, 1997, 1145-1147).
According to a first aspect of the present invention there is provided a method for preparing nanoparticles for use, from a mixture of nanoparticles with another material, the method comprising washing the mixture with a solvent in which the nanoparticles are soluble to remove the said other material and form a solution of nanoparticles in the solvent. The solvent is preferably one in which the said other material is not soluble. Alternatively, the solvent may be one in which the other material is soluble but the nanoparticles are not.
The method preferably comprises separating at least a first fraction of the nanoparticles from a mixture of the solvent and the said other material. These separated nanoparticles may then be used, for example in the applications described below. These nanoparticles are preferably only weakly bound (e.g. unaggregated or only weakly aggregated), and not strongly bound, so that they suitably exist in a disaggregated state. This can assist in subsequent processing steps, such as forming a substantially uniform dispersion of the nanoparticles in another material. The solvent is preferably one that is capable of holding the dissolved nanoparticles in a disaggregated state.
The method preferably includes a step of maintaining the pH of the solvent at a predetermined level. This suitably maintains a charge on the nanoparticles. An acid or a base and/or a suitable buffer may be added to the solvent to maintain the desired pH.
The method preferably includes dialysis through a suitable membrane to remove soluble low molecular weight material (for example surfactant molecules) from the nanoparticle solution. Continuous or intermittent sonification could be performed during dialysis.
The separation may be performed by filtration and/or dialysis and/or centrifugation. Preferably the separation step or another step of the method also allows for the separation of the said first fraction of the nanoparticles from another fraction of the nanoparticles. For example, the nanoparticles of the said other fraction may be a set of particles that are relatively small in comparison to the nanoparticles of the first fraction. Thus, the separation step may also serve to narrow the size distribution of the retained nanoparticles.
The said other material may be a by-product of the formation of the nanoparticles, (examples include reaction products from the formation of the nanoparticles) and materials used to maintain process conditions during the formation of the nanoparticles, such as surfactants which could have been used in a microemulsion process for forming the nanoparticles.
The solvent may be an organic or an inorganic solvent. The solvent may be a polar solvent such as water or methanol (which may improve the solubility of the nanoparticles with polar surfaces) or a non-polar solvent. The solvent may be a polar non-hydrogen bonding solvent. It is preferred that the said other material is soluble in the solvent, and most preferably that the solvent is one in which the said other material is preferentially soluble to the nanoparticles. This can assist in the separation of the components.
The size range of the nanoparticles is preferably, but not necessarily, within the range from 1 nm to 100 nm. Preferably all or substantially all of the nanoparticles are smaller than 50 nm, 30 nm or 10 nm in diameter. Preferably all or substantially all of the nanoparticles are larger than 1 nm, 5 nm or 10 nm in diameter.
The method preferably comprises surface modifying the nanoparticles. This may suitably be achieved by adsorbing a material to the surface of the particles. The material may be added as surface modifying agent to the solution of nanoparticles. The material may, for example, be a silylating agent or a dye or a chemical functional material. The material may promote specific interactions with other materials such as polymers. Alternatively, or additionally, the nanoparticles may already have a surface coating. This may be a coating of a surfactant.
The nanoparticles may be of metallic, semiconducting or insulating material. Examples of suitable materials include inorganic oxides such as SiO2, TiO2, Al2O3 or ZrO2, or ternary or other binary inorganic materials such as BaSO4, YbF3, ZnS or other organic materials, especially polymer materials, such as PTFE, polymethytmethacrylate (PMMA) or polystyrene (PS). The nanoparticles are preferably light transmissive and most preferably optically transparent. Therefore, the material of which the nanoparticles are formed is preferably a wide optical bandgap material.
The nanoparticles may have been formed by any suitable route. Examples include the microemulsion route and the sol-gel route.
A further step in the processing of the nanoparticles is preferably to incorporate them into a body of material. To achieve this the material of the body, or a precursor of it, is preferably added to the solution of nanoparticles. A uniform (or substantially uniform) non-aggregated (or substantially non-aggregated) dispersion of the nanoparticles in the final body is achieved by ensuring that they are held in a substantially disaggregated state until fixed in place in the body, e.g. by removal of the solvent by a step such as evaporation, it is therefore preferable that the material of the body (or its precursor) is soluble in the solvent in which the nanoparticles are dissolved, and does not have undesirable interactions with the nanoparticles that may lead to severe aggregation or phase separation.
The material of the body could be (but need not be) an organic material. Examples are polymers, oligomers and materials of small organic molecules. If the material is a polymer material it may be a conjugated polymer such as poly(p-phenylenevinylene) (PPV). Alternatives include poly(2-methoxy-5-(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (xe2x80x9cMEH-PPVxe2x80x9d), a PPV-derivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers, poly (2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) (xe2x80x9cTFBxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methyoxyphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene)) (xe2x80x9cPFMxe2x80x9d), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl) imino)-1,4-phenylene-((4-methoxyphenyl)imino)-1 ,4phenylene)) (xe2x80x9cPFMOxe2x80x9d), F8 or F8BT. Alterative materials include organic molecular materials such as Alq3. The material is suitably light-transmissive and/or light-emissive.
The presence of the nanoparticles (including optionally any material attached to the surface of the nanoparticles) in the material of the body preferably influences at least one material property of the material of the body. This could be an optical property such as refractive index or an electrical property such as conductivity. Thus the nanoparticles could be dispersed in the material of the body to tailor its refractive indexxe2x80x94either increasing or decreasing it depending on the relative refractive indices of the material of the body and the nanoparticles. The presence of the nanoparticles, and the interaction between the nanoparticles and the polymer (including optionally any material attached to the surface of the nanoparticles) in the material of the body could also influence the morphology of the material of the body, for instance by inhibiting crystallisation.
The volume fraction of the nanoparticles in the body is preferably greater than 1, 5 or 10 volume %. The volume fraction of the nanoparticles in the body is preferably less than 50, 30 or 30 volume %. The density of the nanoparticle distribution in the body is preferably greater than 10xe2x88x9217 and/or less than 10xe2x88x9219/cm2.
It is preferred that the nanoparticles exist in the body in a disaggregated state. This suitably promotes uniformity of the properties in the body. Furthermore, in some circumstances aggregates of the particles could scatter incident light, and it is preferred that the particles are of a sufficiently small size and disaggregated nature that they substantially do not scatter incident light.
The body could be a layer of a device such as an electronic and/or optical device. Preferred non-limiting examples of such devices are as follows:
1. A device comprising a stack of layers defining a light-reflective structure, with at least one of the layers comprising a dispersion of nanoparticles as described above. Preferably alternating layers of the device comprise a dispersion of nanoparticles as described above. This device is suitably a distributed Bragg reflector.
2. A light emissive device in which a light emissive layer or a layer adjacent to the light emissive layer comprises a dispersion of nanoparticles as described above. In this case the nanoparticles may carry a fluorescent dye (suitably as a surface layer).
This dye can suitably be stimulated to fluoresce by energy transfer or light emission from the emissive layer; thus the dye can act to modify the colour of light emission from the emissive layer. The device may also comprise a waveguide structure defined by a relatively high refractive index layer located between two relatively low refractive index layers. One of those three layers preferably comprises a dispersion of nanoparticles as described above which suitably modifies its refractive index to help define the waveguide structure. The waveguide is preferably located outside and/or separately from and/or independent of the light-emissive region of the device (the energy level profile of the device may be arranged to encourage light emission other than in the waveguide). This can permit independent tuning of the material properties of the emissive layer and the waveguide layer. The device may have a pair of mirrors located on either side of itxe2x80x94either mirrors of the type described for device 1 above or mirrors of another type such as cleavage surfaces. These mirrors may define a microcavity which can spectrally redistribute light generated by the device. The device may be a laser, for instance a microcavity or waveguide laser. A mirror could be provided by a DBR grating superimposed on the waveguide structure (on a substrate or any subsequent layer).
Thus according to another aspect of the present invention there is provided a reflective structure (preferably a distributed Bragg reflector) comprising a plurality of layers having alternating refractive indices, each layer comprising a substantially uniform dispersion of nanoparticles. According to another aspect of the present invention there is provided a light-emitting device comprising a light-emitting layer comprising a substantially uniform dispersion of fluorescent nanoparticles. The light-emitting layer preferably comprises an organic material, most preferably a conjugated polymer material. According to another aspect of the invention there is provided a light-emitting device comprising a light-emitting layer located between two waveguide layers having a lower refractive index than the light-emitting layer, and wherein the light-emitting layer and/or the waveguide layers comprises a substantially uniform dispersion of nanoparticles. It will be appreciated that such devices may include any suitable additional features as described herein.
Other aspects of the present invention include any or all of the articles described above. For example, according to a further aspect of the present invention there is provided an optical and/or electronic device including any of the features described above. According to a further aspect of the invention there is provided a method for forming an optical and/or electronic device including any of the features described above. According to a further aspect of the invention there is provided a solution of nanoparticles in other than a strongly bound state, suitably including any of the features described above. According to a further aspect of the invention there is provided a solution of a polymer material (or a polymer precursor material) and nanoparticles in other than a strongly bound state. According to a further aspect of the invention there is provided an organic material containing a substantially uniform dispersion of nanoparticles; preferably the organic material is a semiconductive and/or a polymer material. According to a further aspect of the invention there is provided a method for tailoring at least one property of an organic material, the method comprising forming a substantially uniform dispersion of nanoparticles in the material.
According to a further aspect of the present invention there is provided an optical device comprising a reflective structure having a plurality of layers, each layer comprising a semiconductive conjugated polymer, the refractive indices of adjacent ones of the layers being different. At least one of the layers may comprise a substantially uniform dispersion of light transmissive nanoparticles. At least one of the layers may comprise a partially doped semiconductive conjugated polymer. The semiconductive conjugated polymer is preferably doped in an amount less than one tenth of that which would be required to fully dope the semiconductive conjugated polymer. The plurality of layers may define a distributed Bragg reflector or another photonic structure. The device may comprise a light-emitting region and a pair of electrodes arranged so that the said plurality of layers and the light-emitting region are located between the electrodes. Then, the light-emitting region may be driven to emit light by the application of a voltage between the electrodes, current flowing through the said plurality of layers.
One or more layers of an optical device according to an aspect of the present invention may be formed by a method of forming a partially doped polymer material that comprises: adding a doping agent to the polymer or a precursor thereof, the doping agent being capable of bonding to the precursor or the polymer chain; and causing the doping agent to leave the precursor or the polymer chain to form a dopant capable of doping the polymer chain; wherein fewer moles of the doping agent are added than would be numerically sufficient to fully dope the polymer chain, thus forming a partially doped polymer material. The doping agent may bond to the precursor or polymer chain by replacing a leaving group on the chain. The step of causing the doping agent to leave the polymer chain may be effected by heating. The step of causing the doping agent to leave the polymer chain may result in conjugation of the polymer and/or formation of the polymer from the precursor. The method preferably also includes the step of causing the dopant to dissociate, for example by the application of light and/or heat. The conductivity of the polymer after doping is suitably less than 10xe2x88x923 S/cm, preferably less than 10xe2x88x924 S/cm or 10xe2x88x925 S/cm, and most preferably in the range from 10xe2x88x929 S/cm to 10xe2x88x9213 S/cm . The amount of dopant that is added is preferably an effective amount to achieve a conductivity in such a range. The polymer after doping is preferably partially conducting. The polymer after doping is preferably partially semi-conducting. A photonic device formed in this way may include a plurality of layers of such doped materials, the layers alternating in their levels of doping. The device could be a mirror, for instance a distributed Bragg reflector.